Temperature-Dependent Deformation Behavior of Al-Mg-Sc Alloys Fabricated by Multi-Directional Forging at Room Temperature

Ultrafine-grained Al-Mg-Sc alloys were fabricated by multi-directional forging (MDF) with different number of forging passes of 3, 9 and 15, i.e., to cumulative strains of ΣΔε = 1.2, 3.6 and 6.0, at room temperature. The achieved average grain sizes were 950, 680 and 360 nm at 3, 9 and 15 passes, respectively. Peak-aging treatments at 473 K for 172.8 ks were adopted for a portion of specimens after MDF in order to obtain finely dispersed Al₃Sc precipitates. Grain coarsening did not take place in all the specimens during the aging. The activation volume for plastic deformation was estimated from the strain-rate jump tensile tests before and after the aging. Aging-free 3- and 9-pass specimens showed positive temperature dependence of the activation volume, while that of the aging-free 15-pass one bearing the smallest grain size exhibited a negative temperature dependence. Contrary to these results, values of the activation volume in the peak-aged specimens were approximately identical regardless of grain size or deformation temperature. These results strongly suggested that, due to the precipitation of Al₃Sc, the rate-controlling process of deformation was changed from interaction between forest dislocations and mobile dislocations for the aging-free 3- and 9-pass specimens to interaction between mobile dislocations and Al₃Sc precipitates, or from bowing-out of dislocations from grain boundaries for the aging-free 15-pass specimen to the interaction between the mobile dislocations and precipitates.